This invention relates to counterbalance devices for negating the force of gravity and more particularly to a spring counterbalance mechanism.
Equipment consoles for housing electronic test equipment or computer-display equipment often have a horizontal shelf for holding a keyboard and other operator interface controls or display equipment. Such a shelf may be adjustable in the vertical direction and such adjustment is usually manual by releasing a latch and lifting or lowering the shelf on rolling bearings. Manual operation of such a shelf eliminates the costs associated with drive motors, controls and safety interlocks. The weight of such a shelf may be approximately 125 lbs. and it may be desireable to counterbalance it to within 5 lbs. to minimize operational effort during vertical travel of the shelf.
Several counterbalance devices exist in the prior art for negating the force of gravity. For example, a gas spring uses a pressurized gas in a cylinder to store energy; as a piston rod moves axially, it acts like a compression spring. The gas spring stores more energy than an equal size steel spring, but has seal drag, leakage and temperature sensitivity. Also, because it is an axial device, it must be used with a linkage to compensate for force change over its stroke. Furthermore, any type of axial spring, whether steel coil or gas offers no guidance for shelf motion. Common counterbalance applications include automobile hoods, drafting boards and office chairs. However, the residual imbalance plus friction for these commercial devices is more than 5 lbs. and degradation of the counterbalance effect over time is undesireable. The optimum counterbalance mechanism must result in a shelf that is smooth and quiet in operation and that is stiffly restrained in all six "degrees-of-freedom" to provide good "feel" to the user.